What do humans know up to now about the universe?
What are galaxies?
What is a planet?
How big is the Universe?
What is the altitude above earth at which satellites can maintain orbits for a reasonable time without falling into the atmosphere?
What is the altitude of an international flight?
What is the altitude of most passenger or military jets?
How is the atmosphere of the earth classified?
What is Space?
Where does the atmosphere end and outer space begin?
Why don't we receive light from all the stars in the universe?
What is wavelength?
What are radio waves?
What is electromagnetic radiation?
What is the electromagnetic spectrum?
How do you make a radio wave?
How is data put on radio waves?
What is bandwidth?
What are uplink and downlink?
What is a satellite?
What are some different kinds of orbits?
How does a spacecraft get to where its going?
What are stars made of?
Is there gravity in space?
What is mass?
Why is mass important?
How does propulsion work?
What is in space?
How far away is Earth from the sun?
What is an orbit?
How do we put spacecraft into orbit?
How deep is the ocean?
How do we know a spacecraft's location?
What are planets made of?
What is interstellar dust?
Does heat travel differently in space than it does on Earth?
How can a spaceship leave orbit?
How does gravity work in space?
How do objects in space travel?
How do spacecraft use an orbit to move from planet to planet?
How and why do we control heat on a spacecraft?

What do humans know up to now about the universe?
Earth's location in the universe
Galaxies
Intergalactic Space
List of galaxies
Planets
Planet Earth
Solar System
Stars

What are galaxies?
Galaxies are huge collections of stars, dust and gas. They usually contain several million to over a trillion stars and can range in size from a few thousand to several hundred thousand light-years across. There are hundreds of billions of galaxies in the Universe. Galaxies come in many different sizes, shapes and brightnesses and, like stars, are found alone, in pairs, or in larger groups called clusters. Galaxies are divided into three basic types: spirals, ellipticals and irregulars.

Solar System

A solar system is the collection of planets, asteroids, and moons (moons orbit the planets) that occupy a space around a sun and are held within the gravitational pull of the Sun. Our solar system consists of eight planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune) and a planetoid, Pluto.

What is a planet?
There are 8 planets, a small number of dwarf planets, and a large number of minor planets. The decisive criterion in the definition of a planet is based on dynamics (i.e. the science of forces and motions). This is an intelligent decision, and a decision that we can feel good about, for several reasons:
Planets have always been defined by their dynamics. The word "planets" comes from the Greek word for "wanderer", an object that moves across the background of the fixed stars.
Moons have always been defined by their dynamics. A moon is an object that moves around (orbits) a planet. There are moons that look round and there are moons that don't look round at all. Their shape is irrelevant to the classification.
A classification based on dynamics is far easier to implement than a classification based on physical properties. Newly discovered objects can be classified immediately, long before the details of their physical properties are known.

How big is the universe?
There are no words or numbers (figures) to describe the exact dimensions of universe.
Nobody really knows how big the Universe is because we cannot see to the edge of it. We don't even know if it has an edge. We can only see out to a distance of about 14 billion light years from Earth. This means that the size of the Universe that we can see is about 28 billion light years in diameter (across). Light has not reached us from beyond this distance. In addition, the size of the Universe is changing and gets larger with time.

What is the altitude above earth at which satellites can maintain orbits for a reasonable time without falling into the atmosphere?
100 miles (160 kilometers) above the surface approximately.

What is the altitude of an international flight?
The flight level of an aircraft is based on the purpose of the flight, weather, flight distance, and instructions from supervisors of the aircraft.

As a rule, the longer the flight, the higher the altitude.

What is the altitude of most passenger or military jets?
Most passenger jets fly around only 30,000 feet (5.7 miles). Even the best military jets rarely climb above 100,000 feet (a little less than 19 miles).

How is the atmosphere of the earth classified?
Troposphere
Stratosphere
Mesosphere
Thermosphere
Exosphere

What is Space?
Space is a term that can refer to various phenomena in science, mathematics, and communications.

In astronomy and cosmology, space is the vast 3-dimensional region that begins where the earth's atmosphere ends. Space is usually thought to begin at the lowest altitude at which satellites can maintain orbits for a reasonable time without falling into the atmosphere. This is approximately 160 kilometers (100 miles) above the surface. Astronomers may speak of interplanetary space (the space between planets in our solar system), interstellar space (the space between stars in our galaxy), or intergalactic space (the space between galaxies in the universe). Some scientists believe that space extends infinitely far in all directions, while others believe that space is finite but unbounded, just as the 2-space surface of the earth has finite area yet no beginning nor end.

In mathematics, space is an unbounded continuum (unbroken set of points) in which exactly three numerical coordinates are necessary to uniquely define the location of any particular point. It is sometimes called 3-space because it contains three distance dimensions. If a continuum requires fewer or more than three coordinates (dimensions) to uniquely define the location of a point, that continuum is sometimes called n-space or n-dimensional space, where n is the number of dimensions. Thus, for example, a line constitutes 1-space and a plane constitutes 2-space. When time is considered as a dimension along with the usual three in conventional space, the result is sometimes called 4-space, 4-dimensional space, time-space, or space-time.

In digital communications, the term space refers to an interval during which no signal is transmitted, or during which the signal represents logic 0. The term space may also be used in reference to the time interval separating two characters, bytes, octets, or words in a digital signal.

Where does the atmosphere end and outer space begin?
Very roughly, it starts about 100 kilometers above the Earth, but there is still part of the Earth's atmosphere even at this altitude.

As we said, space is the area above the Earth's atmosphere. However, there is no specific boundary because the atmosphere gradually thins as you move away from the Earth and you can find traces of the gasses we breathe over 100 miles above the earth. In contrast, most passenger jets fly around only 30,000 feet (5.7 miles). Even the best military jets rarely climb above 100,000 feet (a little less than 19 miles).

Why don't we receive light from all the stars in the universe?
There are about 6000 stars that are clearly visible to the naked eye above Earth. Yet we know that there are millions of stars in the universe. Since all stars are putting out light and other kinds of electromagnetic radiation and since light can travel for huge distances in space, why can't we see all the stars?

All stars, like our Sun, send out a huge amount of electromagnetic radiation, including light. However, that light spreads out with distance, making it so that only a tiny fraction reaches us. In addition, depending on the temperature of the star, the main color of light sent out by the star changes. Cooler stars put out redder light, hotter stars put out blue or white light. Generally, colors like white or blue are stronger colors (shorter wavelength, higher frequency) and can be seen more easily at far distances than reds, oranges or yellows. Also, some stars are simply larger than others and send out more light.

What is wavelength?
Forms of electromagnetic radiation like radio waves, light waves or infrared (heat) waves make characteristic patterns as they travel through space. Each wave has a certain shape and length. The distance between peaks (high points) is called wavelength.

What are radio waves?
Radio waves are part of a larger group of waves classified all together as electromagnetic radiation.

What is the electromagnetic spectrum?
What is electromagnetic radiation?
Abbreviated as EM, and also called just spectrum or electromagnetic radiation spectrum, electromagnetic spectrum refers to the complete range of wavelengths of electromagnetic radiation. The electromagnetic spectrum includes the following different types of of radiation (from lowest energy to highest): radio, microwaves, infrared, visible, ultraviolet, X-rays and gamma-rays.

Electromagnetic energy is a term used to describe all the different kinds of energies released into space by stars such as the Sun. These kinds of energies include some that you will recognize and some that will sound strange. They include:
Radio Waves
TV waves
Radar waves (Radio waves or microwaves or
visible light)
Heat (infrared radiation)
Light
Ultraviolet Light (This is what causes Sunburns)
X-rays (Just like the kind you get at the doctor's office)
Short waves
Microwaves, like in a microwave oven
Gamma Rays

How do you make a radio wave?
When a direct electrical current is applied to a wire the current flow builds an electromagnetic field around the wire. This field sends a wave outward from the wire. When the current is removed, the field collapses which again sends a wave. If the current is applied and removed over and over for a period of time, a series of waves is propagated at a discrete frequency. If the current changes polarity, or direction repeatedly, that could make waves, too. This phenomenon is the basis of electromagnetivity and basically describes how radio waves are created within transmitters.

Other kinds of electromagnetic radiation, including radio waves, are made by natural processes such as the nuclear reactions in a star.

How is data put on radio waves?
There are two common ways to put information in a radio wave, and you've likely run into them yourself. They are called A.M. and F.M. just like the two choices you've always known are on a radio. To understand these two ways of sending information it is important to know that radio waves, by themselves, have very regular patterns. Generally they keep the same amplitude or frequency all the time. (Amplitude is the "height" of the radio wave, frequency is how close the waves are to each other.)

What is bandwidth?
Bandwidth is the total range of frequency required to pass a specific signal that has been modulated to carry data without distortion or loss of data. The ideal bandwidth allows the signal to pass under conditions of maximum AM or FM adjustment. (Too narrow a bandwidth will result in loss of data. Too wide a bandwidth will pass excessive noise.)

Transmitters and receivers have bandwidths. The "wider" the receiver's bandwidth is, the more information it can receive on different frequencies.

The term bandwidth is used metaphorically for the carrying ability of Internet carriers. For example, if you can receive information from the Internet over a slow modem, you get less information per second than if you were connected to a fast modem. Thus, you have "low bandwidth" and the Internet appears slower to you.

What are uplink and downlink?
The communication going from a satellite to ground is called downlink, and when it is going from ground to a satellite it is called uplink. When an uplink is being received by the spacecraft at the same time a downlink is being received by Earth, the communication is called two-way. If there is only an uplink happening, this communication is called upload. If there is only a downlink happening, the communication is called one-way.

What is a satellite?
An object in an orbit is called a satellite. A satellite can be natural, like the Moon, or human (or extraterrestrial?) -made. Satellites can travel around planets or around stars such as our Sun.

What are some different kinds of orbits?
Satellites put in space by people serve different purposes. Some use orbits to move from planet to planet. Others stay moving around one planet to do a specific job. The kinds of orbits they travel in help them to achieve this purpose. Some kinds of planetary orbits include:

Geosynchronous Orbits. A geosychronous orbit (GEO) is a circular, low orbit about Earth having a period of 23 hours 56 minutes 4 seconds--that is, the same amount of time it takes for the Earth to turn, so as the Earth spins, the satellite moves in time with it. Geosynchronous means "in time with the Earth." A spacecraft in geosynchronous stays over the same line of longitude. (A line of longitude marks one slice of the world from north to south pole.) Often a satellite in geosynchronous orbit stays above the same spot on Earth. When it does, it is called geostationary. This orbit is ideal for certain kinds of communication satellites, or meteorological (weather) satellites that have a job to do over one part of the world.

Polar Orbits. Polar orbits are useful for spacecraft that carry out mapping or surveillance operations. A satellite in polar orbit goes around the Earth from pole to pole. The planet spins underneath it as the satellite goes from north to south. This gives the spacecraft access to virtually every point on the surface. The Magellan spacecraft used a nearly-polar orbit at Venus. When the planet rotated once, all 360 degrees longitude had been exposed to Magellan's surveillance.

Walking Orbits. There are some things that interfere with making spacecraft follow perfect orbits easily. Planets are not perfectly spherical (ball shaped) and they do not have evenly distributed mass. Some parts of the planet might weigh a little more than others. For example a huge iron concentration could be in one part of the planet, making that side weigh a little more. Also, gravity can be uneven in space. Other bodies such as the Sun, the moon or other satellites, pull on spacecraft in orbit about a planet. Sometimes, scientists choose the path of a spacecraft's orbit to use this other gravity to slowly change the orbit over time. The result is called a walking orbit.

Suns-Synchronous Orbits. Sometimes a walking orbit can be designed so that the orbit changes slowly in time with the planet moving around the Sun, and in time with the planet's rotation so that the spacecraft is always at the same angle to the Sun. This is called a Sun-synchronous orbit. On Earth, this would work out so that the orbit always passes a low point at the same local time every day. This can be useful if instruments on board depend on a certain angle of solar illumination on the surface. Mars Global Surveyor's intended orbit at Mars is a 2-PM Mars local time Sun-synchronous orbit.

How does a spacecraft get to where its going?
Much of the work of getting a spacecraft to its destination is done before it is launched. All objects in the solar system are constantly moving. Scientists must know the clockwork of the solar system well enough to predict where a spacecraft's destination will be, when to launch and how fast to go to meet it in space. In addition to the movement of the objects in the solar system, scientists must take gravity in account. Gravity exerted by large bodies like planets and the Sun in the solar system will "bend" the flight of a spacecraft. If a flight is planned carefully, a spacecraft can use the gravity of planets and moons to do a swingby or be pulled into orbit.

Much of the "aiming" of spacecraft is done at or near launch, when the huge launch vehicle that puts it into space can push it onto a course that will take it to the right place. Once a spacecraft is in flight, small course corrections can be performed.

What are stars made of?
Basically, stars are big exploding balls of gas, mostly hydrogen and helium. Our nearest star, the Sun, is so hot that the huge amount of hydrogen is undergoing a constant star-wide nuclear reaction, like in a hydrogen bomb. Even though it is constantly exploding in a nuclear reaction, the Sun and other stars are so large and have so much matter in them that it will take billions of years for the explosion to use all the "fuel" in the star. The huge reactions taking place in stars are constantly releasing energy (called electromagnetic radiation) into the universe, which is why we can see them and find them on radio telescopes such as the ones in the Deep Space Network (DSN). Stars, including the Sun, also send out a solar wind and burst out occasional solar flares.

Scientists think that the core of the Sun is a 15 million degree Celsius plasma, a soup of electrons and protons that are stripped from hydrogen atoms. This "soup," called plasma, makes up 90 percent of the Sun. Every second, thousands of protons in the Sun's core collide with other protons to produce helium nuclei in a nuclear fusion reaction that releases energy. Just outside the core, energy moves outward by a process called radiation. Closer to the surface, the energy moves out by a process called convection - hot gases rise, cool, and sink back down again. As these masses of gas move, they push off of each other causing "Sun-quakes." These make the material in the Sun vibrate. These Sun-quakes help scientists determine the Sun's internal structure and the processes occurring at different locations underneath the Sun's surface.

Is there gravity in space?
There is gravity everywhere. It gives shape to the orbits of the planets, the solar system, and even galaxies. Gravity from the Sun reaches throughout the solar system and beyond, keeping the planets in their orbits. Gravity from Earth keeps the Moon and human-made satellites in orbit.

It is true that gravity decreases with distance, so it is possible to be far away from a planet or star and feel less gravity. But that doesn't account for the weightless feeling that astronauts experience in space. The reason that astronauts feel weightless actually has to do with their position compared to their spaceship. We feel weight on Earth because gravity is pulling us down, while the floor or ground stop us from falling. We are pressed against it. Any ship in orbit around the Earth is falling slowly to Earth. Since the ship and the astronauts are falling at the same speed, the astronauts don't press against anything, so they feel weightless.

What is mass?
We use the word mass to talk about how much matter there is in something. (Matter is anything you can touch physically.) On Earth, we weigh things to figure out how much mass there is. The more matter there is, the more something will weigh. Often, the amount of mass something has is related to its size, but not always. A balloon blown up bigger than your head will still have less matter inside it than your head (for most people, anyhow) and therefore less mass.

The difference between mass and weight is that weight is determined by how much something is pulled by gravity. If we are comparing two different things to each other on Earth, they are pulled the same by gravity and so the one with more mass weighs more. But in space, where the pull of gravity is very small, something can have almost no weight. It still has matter in it, though, so it still has mass.

Why is mass important?
Mass is important because of two major factors affecting how things move in space: inertia and gravity. The more mass something has, the more of both it experiences. That is why heavy things (things with a lot of mass) are hard to move. When an object is sitting still, it resists moving, and the more mass it has the more it resists. The amount of thrust needed to move something and how fast it ends up moving are both directly tied to its mass. On the other hand, once something massive starts moving, it is very hard to stop. This is also due to the relationship between mass and inertia.

Gravity is also proportional to how much mass each thing has. The bigger an object is, the larger the gravitational pull it exerts.

Because of gravity and inertia, the more massive something is, the harder it is to get into space, the harder it is to keep it there, and the harder it is to move it where you want it to go when it is there. For that reason, one of the goals of the New Millennium project is to make lightweight spacecraft.

How does propulsion work?
Propulsion moves things like spacecraft or jet planes forward by pushing something out of the back. Think of a balloon that you blow up and then release. The air rushing out of the back pushes the balloon forward.

What is in space?
The universe contains every substance and every kind of energy we know, and yet huge portions of it are completely empty. What is around you while you are in space depends on where you are in space. If you are on the surface of a planet, your experience is very different than if you are in a nebula, a meteoroid field or next to a star.

How far away is Earth from the sun?
The sun is at the heart of the solar system. All of the bodies in the solar system — planets, asteroids, comets, etc. — revolve around it. The distance from Earth to the sun is called an astronomical unit, or AU, which is used to measure distances throughout the solar system. The AU has been defined as 149,597,870,700 meters (92,955,807 miles).

Astronomers use the AU for measuring distances throughout the solar system. Jupiter, for example, is 5.2 AU from the sun. Neptune is 30.07 AU from the sun. On the outer edges of the solar system, the Oort Cloud, where comets are thought to originate, is 100,000 AU from the sun. The distance to the nearest star, Proxima Centauri, is about 250,000 AU. However, to measure longer distances, astronomers use light-years, or the distance that light travels in a single Earth year, which is equal to 63,239 AU. So Proxima Centauri is about 4.2 light-years away.

What is an orbit?
An orbit is a regular, repeating path that an object in space takes around another one. An object in an orbit is called a satellite. A satellite can be natural, like the moon, or human (or extraterrestrial?) -made.

In our solar system, the Earth orbits the Sun, as do the other eight planets. They all travel on or near the orbital plane, an imaginary disk-shaped surface in space. All of the orbits are circular or elliptical in their shape. In addition to the planets' orbits, many planets have moons which are in orbit around them.

How do we put spacecraft into orbit?
Spacecraft like weather satellites and the Hubble Telescope need to be lifted most of the distance from the ground to their orbit. One way for them to get there is inside the nose cone of a rocket. Once the rocket reaches an altitude near the satellite's orbit height, the satellite is ejected from the rocket's nose cone and the rocket falls back to Earth, burning up upon reentering our atmosphere (so don't worry about getting hit on the head with bits and pieces of used rockets).

Spacecraft like the Hubble Telescope are lifted into orbit by the Space Shuttle. In that case, the space shuttle itself is lifted by rockets into orbit. The spacecraft to be deployed is riding snugly in the cargo bay. At a certain height, the spacecraft is ejected and small rockets on it move it to the proper orbit altitude.

How deep is the ocean?
The average ocean depth is 4.3 kilometers (2.65 miles)

The average depth of the ocean is about 4,267 meters (14,000 feet). The deepest part of the ocean is called the Challenger Deep and is located beneath the western Pacific Ocean in the southern end of the Mariana Trench.
Challenger Deep is approximately 11,030 meters (36,200 feet) deep.

How do we know a spacecraft's location?
The distance between Earth and the ship is measured by sending up a radio signal from Earth with a time code on it. The spacecraft "bounces" back the signal, and people on the ground can see how long it took to travel from Earth to the ship and back. Since all radio waves travel at the speed of light, scientists can look at how long it took for the signal to make it to the ship and back and figure out the distance it traveled. The angle that the radiotelescope is pointing when it receives the signal tells the direction of the ship.

The distances between planets will vary depending on where each planet is in its orbit around the Sun. Sometimes the distances will be closer and other times they will be farther away.

The reason for this is that the planets have elliptical orbits and none of them are perfect circles. As an example, the distance between the planet Mercury and Earth can range from 77 million km at the closest point, to as far as 222 million km at the farthest. There is a huge amount of different in the distances between the planets depending on their position on their orbit path.

The table below shows the eight planets and the average distance between them. The AU column is the distance in astronomical units.

1 AU is the distance from the Sun to Earth, which is 149,600,000 km.

Planet distance table

From

To

AU

KM

MILES

Mercury

Venus

0.34

50,290,000

31,248,757

Mercury

Earth

0.61

91,691,000

56,974,146

Mercury

Mars

1.14

170,030,000

105,651,744

Mercury

Jupiter

4.82

720,420,000

447,648,234

Mercury

Saturn

9.14

1,366,690,000

849,221,795

Mercury

Uranus

18.82

2,815,640,000

1,749,638,696

Mercury

Neptune

29.70

4,443,090,000

2,760,936,126

Venus

Earth

0.28

41,400,000

25,724,767

Venus

Mars

0.8

119,740,000

74,402,987

Venus

Jupiter

4.48

670,130,000

416,399,477

Venus

Saturn

8.80

1,316,400,000

817,973,037

Venus

Uranus

18.49

2,765,350,000

1,718,388,490

Venus

Neptune

29.37

4,392,800,000

2,729,685,920

Earth

Mars

0.52

78,340,000

48,678,219

Earth

Jupiter

4.2

628,730,000

390,674,710

Earth

Saturn

8.52

1,275,000,000

792,248,270

Earth

Uranus

18.21

2,723,950,000

1,692,662,530

Earth

Neptune

29.09

4,351,400,000

2,703,959,960

Mars

Jupiter

3.68

550,390,000

342,012,346

Mars

Saturn

7.99

1,196,660,000

743,604,524

Mars

Uranus

17.69

2,645,610,000

1,643,982,054

Mars

Neptune

28.56

4,273,060,000

2,655,279,484

Jupiter

Saturn

4.32

646,270,000

401,592,178

Jupiter

Uranus

14.01

2,095,220,000

1,301,969,708

Jupiter

Neptune

24.89

3,722,670,000

2,313,267,138

Saturn

Uranus

9.7

1,448,950,000

900,377,530

Saturn

Neptune

20.57

3,076,400,000

1,911,674,960

Uranus

Neptune

10.88

1,627,450,000

1,011,297,430

How far is Neptune from Earth?
The distance from one planet to another is constantly shifting because both bodies are moving through space. When Neptune and Earth line up on the same side of the sun, at their closest, they are only 2.7 billion miles (4.3 billon kilometers) apart. But when the planets are on opposite sides of the sun, they can put as many as 2.9 billion miles (4.7 billion km) between them.